See also:

Removing the mechanical key
Push release catch 1 in the direction of the arrow and at the same time, remove mechanical key 2 from the SmartKey. ...

Problems with the SmartKey
Problem Possible causes/consequences and  Solutions You cannot lock or unlock the vehicle using the SmartKey. The SmartKey battery is discharged or nearly discharged ...

BAS PLUS (Brake Assist System Plus)
BAS PLUS is only available in vehicles equipped with DISTRONIC PLUS. Observe the "Important safety notes" section. At speeds above approximately 20 mph (30 km/h), BAS PLUS assists ...